Effective delivery of therapeutic proteins for clinical use is a challenge to pharmaceutical science. Once in the blood stream, these proteins are constantly eliminated from circulation within a short time by different physiological processes, involving metabolism as well as clearance using normal pathways for protein elimination, such as filtration in the kidneys (e.g., glomerular) or proteolysis in blood. Once in the luminal gastrointestinal tract, these proteins are constantly digested by luminal proteases. The latter can be a limiting process affecting the half-life of proteins used as therapeutic agents in per-oral administration or subcutaneous, intravenous or intramuscular injection. The problems associated with these routes of administration of proteins are known and various strategies have been used in attempts to solve them. In addition, many therapeutic proteins are glycosylated, and production of glycosylated therapeutic proteins can be costly, highly variable and often difficult to achieve. Production of non-glycosylated forms of such proteins is often not possible due to protein degradation.
A protein family that has been the focus of clinical work and effort to improve its administration and bio-assimilation is the cytokine family, which includes erythropoietin (EPO). Recombinantly produced EPO polypeptides have been approved for treatment of variety of anemias, such those caused by renal failure, chronic inflammation, cancer, and AIDS; however, there is still an urgent need for more stable forms of erythropoietin for therapy. Erythropoietin has a relatively short plasma half-life (Spivak, J. L. and Hogans, B. B., Blood 73(1): 90-99 (1989); McMahon, F. G., et al., Blood 76(9): 1718-1722 (1990)); therefore, therapeutic plasma levels are rapidly lost, and repeated intravenous administrations must be made. Since naturally occurring variants can have undesirable side effects in addition to the problems of administration, bioavailability, and short half-life, there is a need to improve properties of EPO for its use as a biotherapeutic agent. Therefore, among the objects herein, it is an object to provide modified EPO polypeptides and other therapeutic polypeptides that have improved therapeutic properties.